In our earlier filed International Patent application No. PCT/GB2009/002325 magnesium alloys are described which have a content of Erbium of up to 5.5% by weight and which demonstrate improvements in processability and/or ductility over known magnesium alloys such as those commercially known as Magnesium Elektron WE43 and WE54. These improved alloys also have equally good corrosion resistance of those known alloys when assessed using a standard salt fog test. Specifically for wrought applications the described alloys consist of:—                Y: 2.0-6.0% by weight        Nd: 0.05-4.0% by weight        Gd: 0-5.5% by weight        Dy: 0-5.5% by weight        Er: 0-5.5% by weight        Zr: 0.05-1.0% by weight        Zn+Mn: <0.11% by weight,        Yb: 0-0.02% by weight        Sm: 0-0.04% by weight,        optionally rare earths and heavy rare earths other than Y, Nd, Gd, Dy, Er, Yb and Sm in a total amount of up to 0.5% by weight, andthe balance being magnesium and incidental impurities up to a total of 0.3% by weight, %, whereinthe total content of Gd, Dy and Er is in the range of 0.3-12% by weight, andwherein the alloy exhibits a corrosion rate as measured according to ASTM B117 of less than 30 Mpy.        
For casting applications the described alloys consist of:—                Y: 2.0-6.0% by weight        Nd: 0.05-4.0% by weight        Gd: 0-5.5% by weight        Dy: 0-5.5% by weight        Er: 0-5.5% by weight        Zr: 0.05-1.0% by weight        Zn+Mn: <0.11% by weight,        optionally rare earths and heavy rare earths other than Y, Nd, Gd, Dy and Er in a total amount of up to 20% by weight, andthe balance being magnesium and incidental impurities up to a total of 0.3% by weight, wherein        
the total content of Gd, Dy and Er is in the range of 0.3-12% by weight, and wherein
when the alloy is in the T4 or T6 condition the area percentage of any precipitated particles having an average particle size of between 1 and 15 μm is less than 3%.
Our earlier application refers to the previous belief held by experts such as King that the behaviour of the heavy rare earths as alloying constituents was essentially the same and that therefore in magnesium alloys such as WE43 heavy rare earths such as Erbium and Ytterbium were interchangeable. Investigations revealed, however, that such prior belief was not well founded, as revealed by the solid solubility values of individual heavy rare earths in magnesium as set out in Table 2 of PCT/GB2009/002325.
Furthermore in our earlier application Gd, Dy and Er were considered to be essentially equivalent, with each being present in an amount of up to 5.5% by weight. However, the solid solubility data in Table 2 of PCT/GB2009/002325 suggested that the use of Er should be more advantageous than the use of Gd and Dy, and further work has now confirmed this.
Further work has also now established that at the temperatures that magnesium alloys are wrought Terbium is almost as soluble in magnesium as Dysprosium, Holmium possesses a solid solubility in magnesium greater than Dysprosium and is almost as soluble as Erbium, whilst Thulium, and especially Lutetium, possess solid solubilities superior to Erbium. Thus in the present invention one or more of the heavy rare earths Ho, Lu, Tm and Tb can replace the Er used in the alloys of PCT/GB2009/002325 either partly or totally.
The solid solubility limits of the heavy rare earths and selected other rare earths in pure magnesium at various temperatures, including room temperature “RT”, is set out in Table 1. It will be appreciated, however, that for magnesium alloys containing other alloying elements these limits will vary.
TABLE 1Atomic numberElementRT300° C.400° C.500° C.71Lu10-1219.5253570Ybca. 00.51.53.369Tm10-1217.621.727.568Er10-1218.52328.367Ho 8-1015.419.424.266Dyca. 51417.822.565Tb1-212.216.721.064Gdca. 03.811.519.263Eu000062Smca. 00.81.84.361Pm————60Ndca. 00.160.72.259Prca. 00.050.20.658Ceca. 00.060.080.2657Laca. 00.010.010.0339Y1-24.26.510.021Scca. 1212.815.718.8
Although for wrought applications, particularly for structural applications, it was considered that rare earths and heavy rare earths other than Y, Nd, Gd, Dy, Er, Yb and Sm could be present in the total amount of up to 0.5% by weight, provided that the alloy exhibited a corrosion rate as measured according to ASTM B117 of less than 30 Mpy, it has been found that when preparing a magnesium of the type described for use as a medical implant, for example picking up the teaching of EP141739 and 1842507 which require the alloy to be wrought, particularly by extrusion, and must meet additional criteria for such medical use, the limits as set out in PCT/GB2009/002325 can and must be adjusted. For example previously it was considered that, because of the need to retain in the alloys of PCT/2009/002325 mechanical properties, particularly tensile strength, equal to or greater than WE43 type alloys, a greater than impurity amount of cerium and lanthanum could be present and no more than an impurities amount of scandium could be tolerated. However, since for medical uses such as bone-replacement implants, such high mechanical properties are of lesser importance than the behaviour of the alloy in a biological setting, for example its biodegradable characteristics, these prior believed limitation and tolerance concerning cerium, lanthanum and scandium are in fact for the present alloys reversed.
Similarly, whereas it had been thought that an alloy's corrosion behaviour in a standard salt fog test could be used as a guide to its behaviour in a corrosion test in simulated body fluid (SBF), this has been found for some alloys of the present invention not to be the case. Whilst for some their salt fog corrosion behaviour is similar to that of the some of the reference alloys, all of the alloys of the present invention have improved resistance to degradation in SBF when compared to that of the known alloys WE43 and WE54.
As explained in PCT/GB/2009/002325, it is also important for good ductility that the occurrence in the alloy of large particles or clusters of particles be controlled.